An amorphous oxide film formed of indium oxide and zinc oxide, or indium oxide, zinc oxide and gallium oxide has attracted attention as a transparent conductive film or a semiconductor film (for example, one used in a thin film transistor or the like) due to visible light transmittance and a wide range of electric properties ranging from a conductor or a semiconductor to an insulator.
As the method for forming a film of the above-mentioned oxide, a physical film-forming method such as sputtering, PLD (pulse laser deposition) and vapor deposition and a chemical film-forming method such as a sol-gel method have been studied. Of these methods, a physical film-forming method such as sputtering has mainly been studied since a film can be formed uniformly in a large area at relatively low temperatures.
When an oxide thin film is formed by the above-mentioned physical film-forming method, it is common to use a target composed of an oxide sintered body in order to form a uniform film stably and efficiently (at a high film-forming speed).
As the target for forming the above-mentioned oxide film (mainly, a sputtering target), studies have been mainly made on one having a composition of a known crystal form such as In2O3(ZnO)m (m=2 to 20), InGaZnO4 and In2Ga2ZnO7 or one having a composition close to that of these crystal forms.
Specifically, a target which is formed of a sintered body of an oxide which comprises mainly In and Zn and contains a hexagonal compound represented by the general formula In2O3(ZnO)m (m=2 to 20) or a target obtained by doping this oxide with at least one kind of an element having a valency of positive trivalency or higher in an amount of 20 at. % or less is disclosed (Patent Document 1).
Further, a target having a crystal structure of a hexagonal compound such as InGaZnO4 and In2Ga2ZnO7 (homologous structure) has been studied (Patent Documents 2, 3 and 4).
Further, studies have been made on development of a target utilizing the properties of a mixture. For example, development of a target formed of a mixture of a hexagonal compound represented by In2O3(ZnO)m (m=2 to 20) and In2O3 or a target formed of a mixture of a hexagonal compound represented by In2O3(ZnO)m (m=2 to 20) and ZnO (Patent Document 1), a target formed of a mixture of a hexagonal compound represented by InGaZnO4 and a spinel compound represented by ZnGa2O4 (Patent Document 5) or the like has been studies.
In addition, Patent Document 6 discloses an oxide represented by InGaO3(ZnO)m (m=1 to 20) such as InGaO3(ZnO)2 and a synthesis method thereof.
As for targets, no studies have been made on other oxides than those having the above-mentioned known crystal form, and various thin films obtained by changing the composition ratio have been only studied. Specifically, a thin film formed by a method in which the composition ratio is adjusted during the film formation by co-sputtering or the like has been studied (Patent Documents 2 and 7).
Regarding sputtering targets containing indium oxide, zinc oxide and gallium oxide, as for targets formed of oxides having substantially a single crystal structure, studies have been made only on targets formed of InGaZnO4 and In2Ga2ZnO7. No studies have been made on sputtering targets having other crystal forms as substantially a single component or the production of a thin film transistor using it.
Meanwhile, as for oxides which do not have known crystal forms, a change in solubility limit or lattice constant of a sintered body obtained by firing powder raw materials has been reported (Non-Patent Documents 1 and 2). Non-Patent Document 2 gives an example in which calculation is made on the assumption that an oxide having a crystal form represented by In1.5Ga0.5O3(ZnO)m is present. However, no specific studies such as the synthesis of this oxide, possibility of applying it to a sputtering target and a possibility as a thin film transistor material are made (Non-Patent Document 2, TABLE IV).